1. Field of the Invention
This invention relates to a high-rigidity, high-melt-viscoelasticity polypropylene for post-processed sheets and blow molding, and a process for producing the same. More particularly, it relates to a polypropylene having high-rigidity and high-melt-viscoelasticity at the same time, obtained by subjecting propylene to a multi-stage polymerization in the presence of a specified catalyst so that the resulting polypropylene can comprise polymer portions of two sections having specified melt flow indexes, and a process for producing the same.
Further the present invention also relates to a polypropylene having a superior sheet-post-processability (often referred to as sheet-formability), and a process for producing the same.
2. Description of the Prior Art
Sheets produced by processing known polypropylene have various drawbacks that at the time of molding by heat for post-processing (or secondary processing), the resulting sheet sags rapidly the ranges of processing conditions are narrow; the molding efficiency is inferior; sag of wide sheets is large; the thickness of post-processed products is liable to be non-uniform; overlying wrinkles are liable to be formed; etc. Thus, only small-type molded products could have been produced. On the other hand, when known polypropylene is used for blow molding, the following problems are raised:
(1) Since the sag of parison at the time of the molding is large, the thickness of the resulting molded products is non-uniform and hence blow molding process can be applied only to small-type products; (2) if a higher molecular weight polypropylene is used for preventing the above-mentioned sag, an inferior fluidity, a large amount of charge at the time of the molding, a great loss of energy and mechanical troubles may be caused and also the resulting molded products have a notable surface roughening to lose their commodity value; etc. In order to improve the above-mentioned sheet-formability and blow moldability of polypropylene, various processes have so far been proposed. For example, Japanese patent publication No. Sho 47-80614/1972 and Japanese patent application laid-open No. Sho 50-8848/1975 disclose a process of blending low density polyethylene or the like with polypropylene. However, molded products produced from such a mixture are liable to cause surface roughening, and in order to prevent it, a powerful kneading is required at the time of melting; thus the process has to be restricted in the aspect of choice of kneaders and power consumption. Further Japanese patent application laid-open No. Sho 56-70014/1981 discloses a two-stage copolymerization process wherein a difference in the molecular weight as well as a difference in the polymer amount are afforded between the polymer portions formed at the respective stages. However, the melt flow characteristics of the copolymer obtained according to the process are insufficient. Furthermore Japanese patent application laid-open No. Sho 55-118906/1980 discloses a process for making definite the relationship between the melt flow ratio and the melt flow rate of polypropylene. In the case of the process, however, the relationship of the melt flow rate with the melt flow characteristics is not taken into consideration, and the swell ratio (SR), too, cannot be primarily determined relative to the extrusion shear rate of polypropylene; hence the process cannot always correspond to the improvement in various processing characteristics relative to the present invention.
Further, since known polypropylene is lower in rigidity and softer than polystyrene, ABS resin or the like, it is impossible to use it as a stock for molded products needing higher rigidity and higher hardness; hence this has caused a serious bottleneck in extending the application fields of polypropylene.
If it is possible to improve the rigidity of polypropylene, it is possible to reduce the thickness of molded products, which is not only effective for resources-saving but also makes its cooling rate at the time of molding higher; hence it is also possible to make the molding rate per unit time higher, which contributes to improving the productivity in molding and processing.
As a known art for improving the rigidity of crystalline polypropylene, for example, there is a process of adding an organic neucleus-creating agent such as aluminum para-t-butylbenzoate, 1,3- or 2,4-dibenzylidenesorbitol, etc. to polypropylene and molding the mixture, but the process has such drawbacks that the cost is high and hence uneconomical, and moreover, the luster, impact strength, tensile elongation, etc. are greatly reduced. As another means for improving the rigidity, there is a process of using various inorganic fillers such as talc, calcium carbonate, mica, barium sulfate, asbestos, calcium silicate, etc., but this process has such drawbacks that the light weight property and transparency specific of polypropylene are not only damaged, but also the impact strength, luster, tensile strength, additive property, etc. are lowered. As a technique of using polypropylene having a higher isotacticity for obtaining high-rigidity molded products (Japanese patent application laid-open No. Sho 55-81125), but polypropylene used therein has an isotacticity in the range of those according to conventional art; hence the effectiveness of improving the rigidity of molded products is still insufficient.
In view of the present status of the above-mentioned known art, the present inventors have made strenuous studies in order to improve sheet formability, blow processability and rigidity of polypropylene to thereby solve the above-mentioned problems of known art, and as a result, have found that when propylene is subjected to a multi-stage polymerization into polymer portions of two sections, in the presence of a specified catalyst; the relationship between the intrinsic viscosities of the polymer portions of the respective sections is regulated within a specified range; and further the amount ratio of the polymer portions of the respective sections is regulated, then it is possible to obtain a polypropylene having a notably high-rigidity and a notably superior sheet-post-processability and blow moldability. Thus the present invention has been completed.
As apparent from the foregoing, an object of the present invention is to provide a polypropylene having a superior sheet-post-processability, blow moldability and high-rigidity, which have been deficient in known kinds of polypropylene, and a process for producing the same. Another object of the present invention is to extend concrete application fields of polypropylene for sheet molded products and blow molded products and make production of molded products of good quality easier.